This invention relates to ammeters, and more specifically relates to a self-powered ammeter.
In accordance with the invention, an input circuit applies the current to be measured to both a metering circuit and a power supply module. The metering circuit measures the magnitude of a selected input current and generates an output voltage indicative thereof. This voltage is applied to a display circuit which preferably, but not necessarily, provides a digital indication of the magnitude of the selected current.
The power supply circuit includes a current transformer having a primary winding connected in series with the metering circuit input. The secondary winding of the current transformer acts basically as a current source into a burden circuit and acts to charge a power supply capacitor through a suitable rectifier circuit, with voltage regulator means being provided to control the capacitor charge in a novel manner. The output voltage of the novel power supply circuit is then used to power both the metering and the display circuit. In this manner, the ammeter is a self-contained, self-powered unit which will provide a visual read-out of the magnitude of the input currents applied to the ammeter. One of several input currents can be measured through a switching circuit which does not open-circuit the current transformer.